Radiographic imaging such as x-ray imaging has been used for years in medical applications and for non-destructive testing.
Normally, an x-ray imaging system includes an x-ray source and an x-ray detector system. The x-ray source emits x-rays, which pass through a subject or object to be imaged and are then registered by the x-ray detector system. Since some materials absorb a larger fraction of the x-rays than others, an image is formed of the subject or object. The x-ray detector may be of different types, including energy-integrating detectors and photon-counting detectors.
A traditional x-ray detector design normally includes, on the top side, an active detector area covered by detector diodes (pixels), e.g. in the form of strips or rectangular or hexagonal areas p-type doped in the case the substrate is a n-type high resistivity material. The top side also includes a so-called junction termination area including a so-called guard.
For maximum sensitivity the highly resistive n-type part of the detector that builds a so called drift region of the PiN diode structure must be totally depleted of charge. This requires applying a voltage of at least 400 Volts for a 500-550 μm thick n-type region without reaching a condition of junction breakdown at the position of the maximum electric field in the structure. Furthermore, the detector must sustain significantly higher voltage to secure tolerance to the positive surface charge which is created as a result of irradiation in the passivating oxide. This is known to increase the electric field at the surface and reduce the breakdown voltage. The function of the junction termination is to spread the electric field along the surface of the detector in order to reduce the electric field strength and to secure the tolerance to the positive oxide charge and long enough lifetime of the detector under irradiation.
There are two main concepts of the junction termination that are applied to PiN diodes and detectors. One is Multiple Floating Field Rings (MFFR) and the second is so called Junction Termination Extension (JTE). The MFFR uses the principle of dividing the applied reverse voltage into small fractions contained in the spaces between the floating rings surrounding the anode (p+ pixels covered area) and the JTE uses the principle of charge neutrality between the dopant charge in the JTE under depletion (negatively charged acceptors) and in the n-type drift region also under depletion (positively charged donors). A characteristic of both techniques is that they use a large area. The very principle of the field reduction is to widen the depletion region width at the surface as compared to that in the bulk of the material. For the required voltages of 400V to 800V the width of the junction termination is between 100 μm and 500 μm including the guard. The floating rings are normally equipped with metal plates helping to avoid the potential crowding at the edges of the pixel diodes. The JTE does not allow or require any metal plates and is normally more space efficient.
The guard is the outermost electrode contacting the outermost p-type doped ring with a function to collect the leakage current from the areas outside of the detector and towards the detector edge. This electrode is normally connected to the ground.
The drawback of the termination is the loss of the active detector area. Also, since many detectors are combined to cover larger area the lost area in each individual detector constitutes “dead” or blind areas in the detector matrix which has a negative influence on the quality of the obtained image.
U.S. Pat. No. 4,377,816 relates to a semiconductor element having at least one p-n junction and provided with zone guard rings for improving the suppression behavior of the p-n junction. The zone guard ring substantially acts as a so called channel stopper (field stop) to prevent the space charge region (electric field) from reaching the edge of the device and thus prevent leakage of current. This represents a simple planar diode without any junction termination and with the only protection of preventing the electric filed from reaching the side wall surface of the device.
U.S. Pat. No. 8,093,624 relates to an avalanche photodiode having a device structure that enables a fill-factor approaching 100% at visible and near-infrared wavelengths, eliminating the need for optical focusing techniques. There is provided an n-type active region and a p-type active region. A first one of the n-type and p-type active regions is disposed in a semiconductor substrate at a first substrate surface. A second one of the n-type and p-type active regions includes a high-field zone disposed beneath the first one of the active regions at a first depth in the substrate, a mid-field zone disposed laterally outward of the first active region at a second depth in the substrate greater than the first depth, and a step zone connecting the high-field zone and the mid-field zone in the substrate. With this configuration, the photodiode structure prevents non-avalanche photoelectron collection by substantially inhibiting photoelectron paths that circumvent the high-field avalanche region of the device. Conventional channel stop regions, provided as p+ regions, are located at the edges of the photodiode. The photodiode may also include a conventional guard ring structure at the periphery of the cathode, laterally surrounding the photodiode cathode, e.g., in a circular configuration. The avalanche photodiode operates at avalanche condition (breakdown) at low voltage, and the issue of terminating the entire array of pixel diodes is not addressed. U.S. Pat. No. 8,093,624 rather concerns the design and configuration of individual photodiodes, where channel stoppers are used to separate individual pixel diodes.
U.S. Pat. No. 9,087,755 relates to an avalanche photodiode that includes an anode and a cathode formed on a semiconductor substrate, where a vertical electrode with limited extension is in operative electrical communication with a buried component of the photodiode. The idea is to replace lateral conventional field plates and guard rings between individual pixel diodes in an array by vertical guard rings or field plates. Isolation between individual pixels is done using trench etching, and the vertical guard or field plate electrodes are deposited/formed using the sidewalls of the trench structure.
The detector is designed for low voltages since they do not contain junction termination in the form of floating rings or JTE, and the vertical guards or field plates are used to protect individual pixel diodes.